1. Field of the Invention
This invention relates, generally, to a coating composition for forming an abrasion-resistant coating for solid substrates such as ophthalmic lens or other optical articles, and more particularly to a monoepoxysilane composition which is easily curable by radiation.
2. Description of Related Art
Synthetic plastic materials have come into wide use as a lightweight, safe material for ophthalmic lens instead of inorganic glass. However, most plastic materials scratch easily, and are susceptible to chemical attack, when compared to inorganic glass optical products.
Thus, protective coatings for plastic substrates which are abrasion resistant are in great demand and of great interest to the art.
The broad application of protective coatings to a lens substrate is known. Typically, these protective coatings are applied to the desired substrate by spin coating, dip coating, spray coating, and flow coating techniques. Vapor coating techniques are also well known, and to a limited extent, certain in-mold techniques have also been practiced.
Many of the prior art abrasion resistant coatings result from thermally activated initiators, which require exposure to elevated temperature in order to fully develop the physical properties of the coating. Unfortunately, these thermally cured coatings do not become tack-free immediately upon being subjected to the elevated temperatures needed for curing. The tack-free state is achieved after the passage of significant time, sometimes hours. This "lag-time" or delay in achieving a tack-free state presents an opportunity for the coating to be marred by various environmental factors, e.g., airborne particulate matter.
In response, the art has utilized radiation-curable abrasion resistant coatings. The technique of radiation cure does result in a coating composition which is tack-free within seconds of irradiation. The current practice is to use radiation for the cure and utilize a free radical polymerization mechanism with various acrylate monomers to achieve the abrasion-resistant coating.
Radiation curable acrylate coatings have drawbacks. Since the acrylate coatings result from free radical polymerization, careful steps must be taken to exclude oxygen during the coating process. If not excluded, oxygen will inhibit the reaction and the coating will have an inferior surface hardness. An oxygen-free environment is expensive and cumbersome to achieve and maintain during the coating process.
It is therefore highly desirable to formulate a coating composition which is radiation-curable and is formulated in a normal environment including air or oxygen, i.e., expensive oxygen removal steps are avoided.
Abrasion resistant coatings resulting from cationic mechanisms are known. Unlike the free radical mechanisms, cationic polymerization is not inhibited by atmospheric oxygen. However, to achieve satisfactory results, some coatings achieved through cationic polymerization still require the acrylate base. Acrylates are notoriously toxic and tend to shrink. This relatively large shrinkage many times results in poor adhesion of the coating to the substrate, and expensive pretreatment of the substrate is required in order to improve the adhesion.
The present invention obtains excellent clarity and physical properties without the use of acrylate-functional silanes or multifunctional acrylate monomers. In the present invention, an abrasion resistant coating which is radiation curable is obtained from a monoepoxysilane by utilizing acidic colloidal silica, a silane crosslinker, and a cationic photoinitiator as the only required initiator.